110 lines
4.7 KiB
HTML
110 lines
4.7 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1987-2016 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation. A copy of
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the license is included in the
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section entitled "GNU Free Documentation License".
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This manual contains no Invariant Sections. The Front-Cover Texts are
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(a) (see below), and the Back-Cover Texts are (b) (see below).
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>The C Preprocessor: Deleted Code</title>
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<meta name="description" content="The C Preprocessor: Deleted Code">
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<meta name="keywords" content="The C Preprocessor: Deleted Code">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Conditionals.html#Conditionals" rel="up" title="Conditionals">
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<link href="Diagnostics.html#Diagnostics" rel="next" title="Diagnostics">
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<link href="Elif.html#Elif" rel="prev" title="Elif">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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blockquote.smallquotation {font-size: smaller}
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div.example {margin-left: 3.2em}
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Deleted-Code"></a>
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<div class="header">
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<p>
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Previous: <a href="Conditional-Syntax.html#Conditional-Syntax" accesskey="p" rel="prev">Conditional Syntax</a>, Up: <a href="Conditionals.html#Conditionals" accesskey="u" rel="up">Conditionals</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Deleted-Code-1"></a>
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<h3 class="section">4.3 Deleted Code</h3>
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<a name="index-commenting-out-code"></a>
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<p>If you replace or delete a part of the program but want to keep the old
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code around for future reference, you often cannot simply comment it
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out. Block comments do not nest, so the first comment inside the old
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code will end the commenting-out. The probable result is a flood of
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syntax errors.
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</p>
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<p>One way to avoid this problem is to use an always-false conditional
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instead. For instance, put <code>#if 0</code> before the deleted code and
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<code>#endif</code> after it. This works even if the code being turned
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off contains conditionals, but they must be entire conditionals
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(balanced ‘<samp>#if</samp>’ and ‘<samp>#endif</samp>’).
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</p>
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<p>Some people use <code>#ifdef notdef</code> instead. This is risky, because
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<code>notdef</code> might be accidentally defined as a macro, and then the
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conditional would succeed. <code>#if 0</code> can be counted on to fail.
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</p>
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<p>Do not use <code>#if 0</code> for comments which are not C code. Use a real
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comment, instead. The interior of <code>#if 0</code> must consist of complete
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tokens; in particular, single-quote characters must balance. Comments
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often contain unbalanced single-quote characters (known in English as
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apostrophes). These confuse <code>#if 0</code>. They don’t confuse
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‘<samp>/*</samp>’.
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</p>
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</body>
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</html>
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